Composite membrane

ABSTRACT

A composite sheet comprises a porous inorganic membrane and a microporous inorganic film overlying a surface of the membrane, where the membrane may be an anodic aluminium oxide membrane having an average pore diameter of 5 to 100 nm, and the microporous film is of substantially uniform pore size and substantially free of cracks and pinholes. The film may be formed by applying a colloidal sol of an aluminium alkoxide, or a solution of a polymeric precursor of a titanium oxide, to a surface of the membrane to form thereon a gel layer, and drying and heating the gel to convert it to a microporous inorganic film.

This is a continuation of application Ser. No. 039,619, filed Apr. 16,1987, now U.S. Pat. No. 4,938,870.

This invention is concerned with composite membranes which are suitablefor separation and filtration processes in the ultrafiltration range.Ultrafiltration is generally accepted to be defined by retention ofmolecules or particles in the size range 20 nm down to 1 nm (1 nm=1nanometer=10⁻⁹ m). An alternative way of expressing this filtation rangeis by converting molecular size to molecular weight: on this basis it isthe retention of species in the molecular weight range of 300,000 downto 300. The main requirements for ultrafilitration membranes are:

Available in suitable size and shape, and sufficiently robust to resistaccidental damage;

Resistant to chemicals and heat of the kind likely to be encountered infiltration processes. In this respect, inorganic membranes are generallysuperior to organic ones.

A rather uniform pore size. Here again, current polymeric organicultrafiltration membranes have rather wide pore size distributions; as aresult their selectivity is low and their molecular weight cutoff is notsharp.

Substantial or ideally complete freedom from cracks or pin-holes.

Having regard to pore size, a high flux (defined as volume passing perunit area per unit time), to permit rapid passage of filtrate. Thisfeature requires not only a large number of pores per unit area, butalso that the length of the narrow pores be as short as possible.

U.S. Pat. No. 3,944,658 describes a sol-gel technique for making aporous alumina product. An aluminium alkoxide is hydrolysed in water andpeptised with acid to form a clear transparent sol. The sol is gelledand the gel dried and calcined to provide the porous product. It isnoted that the sol can be applied as a coating to porous ceramicmaterials, but this aspect is not further described.

Japan Kokai 60-180979 describes a method of manufacturing a ceramicmembrane for separating condensation components. A porous supportstructure is repeatedly impregnated with an alumina sol, dried andfired, to give a product with fine pores down to about 1 nm diameter.

A. F. M. Leenaars and co-workers at Twente University of Technology inHolland have published a series of papers on alumina films produced bysol-gel techniques. An article in Journal of Colloid and InterfaceScience, vol 105, May 1, 1985, 27 to 39 describes the formation ofcomposite membranes by applying alumina sols to porous alpha-aluminasupports. The supports were dipped into the sol for short periods,typically 0.5 seconds and removed, dried and calcined. Calcined layerthicknesses varied from 3 to 8 microns, but crack-free films could onlybe obtained with layers less than 5 microns. The thinnest layers couldonly be obtained by very short dipping times when thicknesses werelargely uncontrolled and highly variable, i.e. plus or minus 0.5microns. Significant impregnations of the support by the sol generallytook place on dipping.

Articles in Journal of Membrane Science, 24 (1985) pages 245-260 and261-270 describe the use of such composite membranes in experiments todetermine their permeability for pure liquids and their ultrafiltrationand hyperfiltration properties. The sol-gel films tested were, aftercalcining, from 2.5 to 5.5 microns thick, as a result of which lowfluxes were generally obtained. Some pin-holing was present in thefilms, as a result of which fluxes were in isolated cases larger.

As they stand, the products described in these articles would not besuitable for use as ultrafiltration membranes. It is an object of thisinvention to overcome these problems and provide composite membranessuitable for use in ultrafiltration.

In one aspect, the invention provides a composite sheet comprising aporous inorganic membrane and a microporous inorganic film overlying asurface of the membrane, which microporous inorganic film is ofsubstantially uniform pore size and substantially free of cracks andpinholes. The porous inorganic membrane may be of alumina, preferably aporous anodic aluminium oxide membrane, which may be symmetric orasymmetric. The porous inorganic membrane may preferably have an averagepore diameter of 5 to 100 nm at the surface overlain by the film. Themicroporous inorganic film preferably has a substantially uniformthickness of from 0.01 to 2.0 microns.

In another aspect, the invention provides a method of making thiscomposite sheet, which method comprises providing a porous inorganicmembrane, applying a colloidal sol of an inorganic material or asolution of a polymeric precursor to an inorganic material to a surfaceof the membrane so as to form a gel layer thereon, and drying andheating the gel to convert it to a microporous inorganic film ofsubstantially uniform pore size and substantially free of cracks andpinholes. The surface of the membrane is preferably pre-treated with asolution of a silicate, borate or phosphate.

Conventional anodic aluminium oxide membranes are symmetric, withgenerally cylindrical pores extending straight through. These aresuitable for use in the invention, but particular advantages can accrueif the membrane is asymmetric.

A suitable asymmetric porous anodic aluminium oxide membrane isdescribed in European Patent Specification 178831. This membrane has asystem of larger pores extending in from a first face, which largerpores may have a diameter (near their inner ends) of some 10 nm to 2microns at a length of from 0.1 to 100 microns; and a system of smallerpores extending in from a second face, the smaller pores having asubstantially uniform minimum diameter of at least 2 nm and preferably 5to 100 nm but less than half the diameter of the larger pores. Theanodic membrane can be made as thick as is desired to provide adequatemechanical strength for the composite membrane. The anodic membrane hasa rather large number of pores per unit area, and these extend generallyperpendicular to the face to which they lead. The region of minimum porediameter of the smaller pores is at or close to the second face of themembrane.

The inorganic membrane may be a refractory membrane. Overlying a face ofthe membrane is a microporous film of an inorganic material which may berefractory, for example a ceramic oxide. It is envisaged that this maybe an oxide of aluminium, titanium, zirconium, silicon, tantalum,cerium, hafnium, yttrium, thorium, tin, germanium, indium vanadium,niobium, iron, chromium, cobalt, boron and combinations thereof.

The thickness of the microporous film is preferably substantiallyuniform over the surface of the membrane, preferably from 0.01 to 2.0microns, particularly from 0.03 to 0.5 microns. The thicker themicroporous film the lower is the liquid flux obtainable through it onultrafiltration, and for this reason a preferred maximum limit is set at2 microns.

The films contain pores of substantially uniform pore size andsubstantially free of cracks and pinholes. The average pore size ispreferably from 0.5 to 30 nm, particularly 1 to 4 nm, and pores morethan twice the average size are preferably substantially absent.

The method of the invention involves applying a colloidal sol of aninorganic material (such as a ceramic oxide) or a solution of apolymeric percursor to an inorganic material (such as a metal oxide) toone or both faces of the membrane. The colloidal sol may be derived byknown means from an inorganic oxide powder, such as an oxide of theelements mentioned above. More preferably the colloidal sol or polymericsolution is derived by hydrolysis of a metal alkoxide. For example,boehmite sols may be prepared using the procedure described in U.S. Pat.No. 3,944,658. According to this procedure, an aluminium alkoxide ishydrolysed in an excess of water maintained at 80° C., and subsequentlypeptized with an acid, to form a clear transparent sol. The sol consistsof colloidal particles of stable crystalline aluminium monohydrate,AlO(OH), dispersed in the aqueous phase. The sol so produced typicallycontains about 30 gl⁻¹ of aluminium values expressed as Al₂ O₃, and canbe adjusted to the desired concentration for coating by dilution withwater or evaporation. Coatings may be achieved with sols ofconcentration up to 190 gl⁻¹, preferably 10 gl⁻¹ to 90 gl⁻¹, ofaluminium values expressed as Al₂ O₃. As an alternative example,--Ti--O--Ti-- polymer solutions may be prepared using a proceduresimilar to that described in the article by C J Brinker and M SHarrington in Solar Energy Materials, 1981, volume 5, 159-172, wherein atitanium alkoxide is partially hydrolysed in an alcoholic solution atroom temperature in the presence of an acid catalyst to form a stable--Ti--O--Ti-- polymer solution. The solution so produced typicallycontains about 10 to 30 gl⁻¹ of titanium values expressed as TiO₂, andcan be adjusted to the desired concentration for coating by evaporationof solvent or further dilution with alcohol. The sol or solution can beapplied to the anodic oxide membrane in the freshly concentrated state,or aged to increase its viscosity prior to application. Control over thefilm thickness in the composite membrane can be achieved in part bycontrol over the concentration and viscosity of the sol prior toapplication.

Prior to sol deposition, it may be advantageous to pre-treat themembrane, particularly an anodic aluminium oxide membrane or otherinorganic membrane, by immersion in a solution of a silicate, phosphateor borate. The effect of this pre-treatment can be to permit thesubsequent formation of a gel layer giving substantially completecoverage of the membrane more easily than is possible without thepre-treatment. The mechanism by which this desirable effect is achievedis not fully understood, but the pretreatment may improve thewettability of the membrane surface, and may deposit on or react withthe surface to form a layer which is receptive to the sol and helps togel it. The solution may be, in order of decreasing preference, anaqueous solution of an alkali metal metasilicate such as sodiummetasilicate, an alkali metal triphosphate particularly pentasodiumtriphosphate, or an alkali metal borate such as disodium tetraborate.The concentration is preferably from 1% up to saturation, typically inthe range 3 to 5 percent by weight. The temperature may be ambient suchas from 10° to 30° C. The period of immersion of the membrane in thesolution is generally in the range 1 second to 10 minutes, and maytypically be around 5 seconds. Immersion time needs to be adjusted, inrelation to the other process variables, to be sufficient to modify thesurface of the membrane, but not so great as to deposit substantialamounts of material in a way that might reduce the diameter of the poresor even block them completely. This should be followed by rinsing themembrane in distilled water.

Addition of a surface active agent to the sol prior to application aidsthe formation of thin uniform gel layers. Non ionic surfactants such asNonidet (Octylphenyl ethylene oxide condensate supplied by BDHChemicals), or Methocel (methyl cellulose polymer supplied by DowChemical Company) added typically at the concentration level of 0.1 to 1percent by weight, can result in thinner uniform gel layers than areobtained with unmodified sol.

In addition, the production of thin uniform films is facilitated by thenature of the porous inorganic membrane. In the case of anodic aluminiumoxide membranes, the surface is smooth and essentially free ofmacro-defects and this results in thin uniform defect-free coatings. Inthe case of the deposition of aqueous boehmite sols onto the asymmetricanodic aluminium oxide membranes, the pore size of the face of themembrane in contact with the sol can be smaller than the crystallitesize of the boehmite particles in the sol, with the consequence thatlittle or no intrusion of the sol into the membrane occurs duringdeposition.

The thin uniform gel films can be formed on the second surface of theanodic oxide membrane by deposition of a concentrated sol followed byair drying. Various techniques including brush, spray, dip, spincoating, electrophoretic and thermophoretic techniques may be used toapply the sol to the membrane. Spray coatings can be applied using anaerosol method. The anodic oxide membrane is suspended vertically topermit excess sol to drain off and spraying is conducted until fullcoverage of the membrane surface is achieved.

To prepare spin coated composite membranes, the membrane can be attachedhorizontally to the platen of a commercially available spin coatingunit. A known quantity of the concentrated sol is applied to themembrane surface and is permitted to reside thereon for a predeterminedperiod of time typically up to 60 seconds. Excess sol is removed byspinning the coated membrane, typically at a speed from 200 to 2000 rpm.The thickness of the gel film is controlled by the sol concentration andage, the residence time of the sol on the membrane surface, and the spinspeed and spin time.

The freshly coated membrane is then heated to transform the gel layerinto a microporous refractory film. For example, heating converts aboehmite gel layer into a mechanically stable gamma-Al₂ O₃ structure.Heating conditions are not critical to the invention and may beconventional, bearing in mind the need to avoid thermal shock whichmight result in the formation of cracks or pin-holes. A typical heatingschedule for a boehmite gel layer might be: (a) heating at 50° C. perhour to 200° C. followed by isothermal treatment at 200° C. for 15minutes, (b) subsequent heating at 200° C. per hour to 450° C. followedby an isothermal hold for 15 minutes, (c) cooling at 50° C. per hour toroom temperature. The first part of the heating schedule up to 200° C.is designed to remove absorbed water; the second stage to 450° C.removes bound water and transforms the gamma-AlOOH to gamma-Al₂ O₃. Thistransformation occurs at temperatures at or above 390° C. During theheating operation, considerable shrinkage of the film takes place. Forfilms of the thickness with which this invention is concerned, shrinkageis mainly in a direction perpendicular to the plane of the film, i.e.the film gets thinner. For thicker films, shrinkage may be in the planeof the film and may result in cracking.

An advantage of using a membrane having small pores, at least at thesurface overlain by the microporous films is that a thin film can bridgethe pores without sagging or bursting and without excessive entry offilm material into the pores. Membranes with pores having averagediameters from 5 to 100 nm are preferred for this reason. However forcertain applications it may be preferred to apply the sol/gel layers tothe large-pored surface of an asymmetric membrane or even to both sides.It is noted that when the application is made to the large-poredsurface, the penetration of the sol is still substantially limited bythe restrictive effect of the small pores at the opposite face.Alternatively it is possible to use a membrane, e.g. an anodic aluminiumoxide membrane, or a tape-cast membrane, which may have larger pores ormay not have been pre-treated with a silicate or other solution, and toachieve complete coverage by a film having the desired microporousstructure by two or more successive sol/gel applications. Thus, a firstaqueous sol may be applied to the membrane and the resulting geloptionally dried and heated; and a second sol, aqueous or alcoholic, maybe applied to the thus-formed gel or microporous film and the resultinggel or gels dried and heated. This technique is particularly suitablefor anodic oxide membranes with parallel pores or for the large poresurface of asymmetric membranes. The membrane is completely covered bytwo superimposed microporous films which may be arranged to have thesame or different pore sizes and which should be completely free ofcracks and pinholes.

In order to prevent ingress of the colloidal sol (or polymer solution)into the pores of the membrane, it is possible to increase itsviscosity. This can be done simply by adding a relatively viscousmiscible organic liquid. Thus for example a relatively high boilingrelatively viscous polyol such as ethylene glycol or glycerol may beadded to an aqueous sol which may thereafter be boiled to remove some orall of the water but without breaking the colloidal dispersion ofinorganic material.

In order to prevent the ingress of the colloidal sol (or solution of apolymeric precursor to an inorganic material) into the pores of themembrane, it is possible to increase its viscosity. This can be donesimply by adding a relatively viscous miscible organic liquid.Alternatively, a relatively viscous relatively high boiling pointliquid, for example a polymer such as polyvinyl alcohol or a polyol suchas ethylene glycol or glycerol, may be added to the colloidal sol (orsolution of a polymeric precursor) and thereafter boiled to remove someor all of the less viscous liquid but without breaking down thecolloidal dispersion or solution of inorganic material.

Reference is directed to the accompanying drawings, in which:

FIG. 1 is a schematic cross section through a composite membraneaccording to one aspect of the invention, and

FIG. 2 is a graph of pore size distribution against pore radius.

Referring to FIG. 1, a composite membrane comprises a porous anodicaluminium oxide membrane and a microporous ceramic oxide film 10. Theporous membrane is asymmetric by virtue of having a system 12 of largerpores extending in from a first face 14 and a system 16 of smaller poresextending in from a second face 18 the system of larger poresinterconnecting at 20 with the system of smaller pores. The figure isnot to scale; the anodic oxide film may typically be 30 microns thickwhile the microporous layer may be 0.3 microns thick.

FIG. 2 shows pore size distributions within the compositeultrafiltration membrane, determined using a combination of multipointBET gas adsorption isotherms (up to 150 A) and mercury porosimetry (from200-1000 A). The 12.5 nm (125 A) and 76 nm (760 A) radius pores observedin the figure are from the anodic membrane support. The gamma-aluminafilm has a narrow pore size distribution with an apparent mean poreradius of about 2 nm. However it is known that the pores are actuallyslit-shaped rather than cylindrical, as assumed in the BET analysis.Therefore the pore diameter of about 4 nm obtained using BET analysis islarger than the actual slit width (2.7 nm) observed and measureddirectly using transmission electron microscopy. It is this dimension,the slit width, which determines the performance of the layer inultrafiltration processes.

EXPERIMENTAL PROCEDURE 1 ALUMINA COATINGS

There follows a description of the preparation and concentration of aboehmite sol.

In a typical experiment, the boehmite sol was prepared by addingaluminium secondary butoxide (1 mole) to deionised water (1.8 l) whichwas heated to a temperature above 80° C. The mixture was stirredvigorously and maintained at 90° C. One hour after the addition of thealkoxide, nitric acid (0.07 mole) was added to peptize the sol. The solwas kept boiling in an open reactor for about 2 hours to evaporate mostof the secondary butanol and then maintained at 90° C. under refluxconditions for a further 16 hours, until a clear transparent sol wasobtained.

Some typical properties of the sol were determined to be:

    ______________________________________                                        Concentration      29.8 gl.sup.-1 Al.sub.2 O.sub.3                            pH                 4.00                                                       Conductivity       2.20 mmhos, 21° C.                                  Viscosity          5 centipoise, 21° C.                                ______________________________________                                    

The pH, conductivity and viscosity of the sol remained unchanged over aperiod of one month when the sol was kept at RT (21° C.).

Boehmite sols so prepared, containing 30 grammes per liter of Al₂ O₃,can be evaporated by heating in air to produce concentrated solscontaining up to 190 grammes per liter of Al₂ O₃, or diluted by additionof water to concentration below 10 gl⁻¹. The sols can either be used forcoating in the freshly concentrated state, or aged to increase theirviscosity prior to coating. The room temperature (21° C.) ageingcharacteristics of a sol concentrated to 62 grammes per liter of Al₂ O₃over a period of one month were determined to be:

    ______________________________________                                        Age of sol           Conductivity                                                                             Viscosity                                     (days)    pH         (mmhos)    (cp)                                          ______________________________________                                         0        3.72       5.11       5                                             10        3.80       4.48       6                                             20        3.83       4.70       7                                             30        3.83       4.44       8                                             ______________________________________                                    

All measurements were conducted at 21° C.

The porous membrane used in the following examples was a porous anodicaluminium oxide film as described in EPA 178831. This material has anasymmetric pore structure with a system of larger pores extending infrom one face and interconnecting with a system of smaller poresextending in from the other. The membrane was used in the form of sheets14 cm×14 cm×30 microns thick with typical internal dimensions:asymmetric layer thickness about 0.5 microns, average large porediameter 150 nm, average small pore diameter 25 nm. For Examples 1, 2and 4 (but not Examples 3 and 5) this membrane was pre-treated byimmersion in a saturated (5%) solution of sodium metasilicate, followedby rinsing in distilled water and drying in air.

The following examples illustrate the invention.

EXAMPLE 1

A boehmite sol aged 3 days having a concentration of 62 grammes perliter Al₂ O₃, viscosity 5 cp (21° C.), and conductivity 5.0 mmhos (21°C.) was deposited on a 14 cm by 14 cm pretreated sheet of the anodicmembrane support by spin coating using a sol residence time of 30seconds, a spin speed of 500 rpm and a spin of 30 seconds. The coatedsupport was heated by a) heating at 50° C. per hour to 200° C. followedby an isothermal hold for one hour, b) heating at 50° C. per hour to450° C., holding at 450° C. for one hour, and c) cooling at 50° C. perhour to room temperature. This produced a composite ultrafiltrationmembrane with a uniform gamma-Al₂ O₃ coating of 0.4 microns thickness,and a permeability for pure water at 110 kPa of 0.010 ml min⁻¹ cm⁻².

EXAMPLE 2

An unaged boehmite sol of concentration 45 grammes per liter, viscosity5 cp, conductivity 3.22 mmhos (21° C.) was spray coated onto avertically suspended 14 cm by 14 cm pretreated sheet of the anodicmembrane support until full coverage of the support by the sol wasachieved. Excess sol was allowed to drain from the support. Heating at450° C. using the same heating rate as in Example 1 produced a compositeultra-filtration membrane with a coating thickness of 0.3 microns and apermeability for pure water at 110 kPa of 0.014 ml min⁻¹ cm⁻².

EXAMPLE 3

An unaged boehmite sol (5 ml) of concentration 62 grammes per liter Al₂O₃, viscosity 5 cp, conductivity 5.1 mmhos, containing <1% octylphenolethylene oxide condensate was deposited on a 14 cm by 14 cm sheet of theanodic membrane support by spin coating using a sol residence time of 30seconds, a spin speed of 500 rpm and a spin time of 30 seconds. Heatingat 450° C., using the same heating rate as in Example 1, produced acomposite ultrafiltration membrane with a thin uniform gamma-Al₂ O₃coating of 0.07 microns thickness. This example shows how the use ofwetting agents can result in thinner films.

EXAMPLE 4

The flow rates reported in Examples 1-3 were determined as follows. Flowrates for pure distilled and deionised water at pressures up to 2×10⁵ Pawere determined for composite ultrafiltration membranes 25 mm or 43 mmin diameter using an Amicon stirred cell (model 8050). Measurements offlow rates were made twenty minutes after the water feed was pressurisedto allow the system to obtain equilibrium. Typical pure water fluxes fora number of other composite membranes according to the inventioncalcined at 450° C. are given in the following table:

    ______________________________________                                                 Film Thickness                                                                            Pore Slit Flux (ml min.sup.-1                            Sample No.                                                                             (microns)   Size (nm) cm.sup.-2) at 110 kPa                          ______________________________________                                        A        0.06        2.7       0.045                                          B        0.43        2.7       0.014                                          C        0.28        2.7       0.026                                          ______________________________________                                    

EXPERIMENTAL PROCEDURE 2 TITANIA COATINGS

There follows a description of the preparation of a partially hydrolysedtitanium alkoxide derived polymer solution which can be deposited upon aporous refractory support and calcined to result in a microporous filmof titania.

In a typical experiment, the polymer solution was prepared by addingtitanium isopropoxide (0.012 mole) to 50 cm³ isopropanol under anhydrousconditions at room temperature. Dionised water (0.017 mole) was added toa second 50 cm³ volume of isopropanol at room temperature. Thewater/alcohol solution was added dropwise while stirring at roomtemperature to the alkoxide/alcohol solution. Partial hydrolysis andpolymerisation of the alkoxide resulted. An acid catalyst, typicallyHNO₃ (0.006 mole) was added to the solution to cause peptization. Aclear solution resulted, of concentration 10 gl⁻¹ of titanium valuesexpressed as TiO₂, of viscosity 3 cp (21° C.).

The titanium polymer solution so prepared can be concentrated to greaterthan 30 gl⁻¹ of titanium values expressed as TiO₂, but is preferablyused for film deposition at a concentration of 10 gl⁻¹ of titaniumvalues expressed as TiO₂.

The following example illustrates the invention for a titania coating.

EXAMPLE 5

A freshly made polymer solution, having a concentration of 10 gl⁻¹ TiO₂,and viscosity 3 cp, was deposited on a 14×14 cm sheet of theunpretreated anodic aluminium oxide membrane by spin coating. A doublecoat was applied, each coat involved a residence time of 5 seconds, aspin speed of 500 rpm and a spin of 30 seconds. The membrane was heatedas follows:

a) heating at 20° C. per hour to 200° C. followed by an isothermal holdfor 10 minutes

b) heating at 50° C. per hour to 400° C., holding at 400° C. for 1minute and

c) cooling at about 200° C. per hour to room temperature.

This produced a composite ultrafiltration membrane with a uniform TiO₂coating of approximately 0.1 microns thickness and a permeability forpure water at 110 kPa of 0.010 ml.min⁻¹ cm⁻².

EXAMPLE 6

This example shows the beneficial effects that can be obtained bypre-treating the membrane.

Sheets of the anodic membrane support of dimensions 14×14 cm wereimmersed in various aqueous solutions of 5% by weight sodiummetasilicate, 5% by weight pentasodium triphosphate or 5% by weightdisodium tetraborate at room temperature for a period of 5 seconds.Following this pretreatment the membrane sheets were rinsed thoroughlyin distilled water, dried in air, and subsequently spin coated withboehmite sol of concentration 30 gl⁻¹ using a residence time of 5seconds and a spin speed of 500 rpm. The effectiveness of thepretreatment was monitored by direct visual observation of the degree ofgel coverage after coating, since uncoated regions of the membraneexhibited different contrast to coated regions when illuminated by whitelight. Use of the pretreatments listed in this example resulted inimproved gel coverage of the membrane compared with untreated membrane.

EXAMPLE 7

A 14 cm×14 cm sheet of the pretreated anodic membrane was attached tothe platten of a commercial spin coating unit and spun at 100 rpm. Aboehmite solution of concentration 27.5 gl⁻¹ was deposited on thespinning membrane by brush coating, followed by spinning at 500 rpm for30 seconds. Heating at 450° C. using the same heating schedule asExample 1, produced a composite ultrafiltration membrane with a thinuniform Al₂ O₃ coating. Scanning electron microscopy was used todetermine the coating thickness at intervals of 5 mm along a 30 mm longsection of the membrane. Over a 20 mm length the thickness variedbetween 0.13 and 0.14 microns (<8%). Over the remaining 10 mm length thethickness was 0.23 microns, the additional thickness being caused bybrush edge effects encounted during brush coating. Oblique angle SEM ofthe composite ultrafiltration membrane, conducted concurrently with thethickness determinations, revealed no evidence of cracks or pinholds inthe coating.

For ultrafiltration, the composite membranes of this invention have thefollowing advantages over prior ceramic oxide materials:

The flux is significantly higher for equivalent pore sizes. This resultsfrom the ability, provided by the fabrication methodology describedherein, of being able to apply a uniform very thin gel film. Ceramicoxide film thicknesses are typically a tenth of those described in theprior art.

They have more uniform thickness and a smoother defect-free surface withabsence of major pinholes and cracks. Variations in film thickness arenot substantially more than 5% according to this invention compared with20% in the prior art. This results in reproducible flow rates. Very thinfilms, down to 0.07 microns or lower, are possible and sol intrusioninto the asymmetric side of the anodic membrane support is negligible.

EXAMPLE 8

A pretreated 14×14 cm sheet of the anodic membrane was spin coated withunaged boehmite solution of concentration 45 gl⁻¹ at 500 rpm for 30seconds. Heating at 450° C. resulted in a uniform coating of Al₂ O₃ 0.2microns in thickness. Scanning electron microscopy of the surface of thecoating revealed less than 5 pinholes of size <1 microns in an area400×100 microns. No pinholes larger than 1 micron in an area wereobserved. Leenars and co-workers in Journal of Membrane Science 24(1985) pages 245-260 reported the occurrence of several (<5) pinholes ofdimensions greater than 10 microns over a membrane surface area of 10cm³, but made no statement concerning the occurrence of smallerpinholes.

EXAMPLE 9

165 g of alumina and 0.42 g of magnesia were slurried in a liquid systemcomposed of 78 g trichloroethylene, 32 g of ethanol, 3.8 g of corn oil,8.4 g of polyvinyl butyral and 14.2 g of polyethylene glycol. The slurrywas tape-cast on a glass substrate into a film of width 173 mm, whichwas dried in air to yield a flexible tape of thickness 0.14 mm. Discs ofdiameter 26 mm were cut from the tape and partially sintered to producea porous ceramic material of average pore size 0.3 μm. The discs werepretreated by immersion for 5 seconds in a 5% solution of sodiummetasilicate, and subsequently spray coated with a viscous boehmite solof concentration 15 gl⁻¹ in which the water had been replaced bydiethylene glycol by adding 100 ml of diethylene glycol to 50 ml of solat a concentration of 30 gl⁻¹ and boiling to evaporate to 100 ml toincrease the viscosity. The coated porous substrate was heated for 1hour at 450° C. to transform the gel layer into a stable γ-Al₂ O₃ filmcontaining pores of slit width 4.2 nm.

An aqueous boehmite sol of concentration 30 gl⁻¹ was subsequentlydeposited upon the first solgel layer by spray coating. This wasfollowed by heating for 1 hour at 450° C., which produced a stable γ-Al₂O₃ film of pore slit width 2.8 nm. The composite ultra filtrationmembrane thus formed had a total coating thickness of 1 μm and a purewater flux at 110 kPa of 0.016 ml min⁻¹ cm⁻².

We claim:
 1. A composite sheet comprising a porous anodic aluminiumoxide membrane and a microporous inorganic film overlying a surface ofthe membrane, wherein the microporous inorganic film is of substantiallyuniform pore size and substantially free of cracks and pinholes.
 2. Acomposite sheet as claimed in claim 1, wherein the microporous film isfrom 0.01 to 2.0 microns thick.
 3. A composite sheet as claimed in claim1, wherein the porous anodic oxide membrane is asymmetric by virtue ofhaving a system of larger pores extending in from a first face and asystem of smaller pores extending in from a second face, the system oflarger pores interconnecting with the system of smaller pores and themicroporous inorganic film overlying the second face.
 4. A compositesheet as claimed in claim 3, wherein the membrane has an average porediameter of 5 to 100 nm at the surface overlain by the film.
 5. Acomposite sheet as claimed in claim 1, wherein the microporous inorganicfilm is of a metal oxide.
 6. A composite sheet as claimed in claim 5,wherein the microporous inorganic film is of gamma-alumina.
 7. Acomposite sheet as claimed in claim 1, wherein the average pore size ofthe microporous film is from 0.5 to 30 nm.
 8. A composite sheet asclaimed in claim 1, wherein the microporous film is from 0.03 to 0.5micron thick.
 9. A composite sheet as claimed in claim 1, wherein theporous anodic oxide membrane is asymmetric by virtue of having a systemof larger pores extending in from a first face and a system of smallerpores extending in from a second face, the system of larger poresinterconnecting with the system of smaller pores, and the microporousinorganic film overlying the first face.
 10. A method of making acomposite sheet, which method comprises providing a porous anodicaluminium oxide membrane, applying a colloidal sol of an inorganicmaterial or a solution of a polymeric precursor to an inorganic materialto a surface thereof so as to form a gel layer thereon and drying andheating the gel to convert it to a microporous inorganic film ofsubstantially uniform pore size and substantially free of cracks andpinholes.
 11. A method as claimed in claim 10, wherein the porous anodicaluminium oxide membrane is asymmetric by virtue of having a system oflarger pores extending in from a first face and a system of smallerpores extending in from a second face, the system of larger poresinterconnecting with the system of smaller pores, the colloidal sol orthe solotion being applied to the second face.
 12. A method as claimedin claim 10, wherein the colloidal sol is of a refractory metal oxide orthe solution is of a polymeric precursor to a metal oxide.
 13. A methodas claimed in claim 12, wherein the colloidal sol is a boehmite solobtained by hydrolysis of an aluminum alkoxide.
 14. A method as claimedin claim 12, wherein the solution of a polymeric precursor to a metaloxide is a solution obtained by partial hydrolysis of a titaniumalkoxide.
 15. A method as claimed in claim 10, wherein the colloidal solor solution contains a surface active agent.
 16. A method as claimed inclaim 10, wherein the colloidal sol or solution is applied to the porousmembrane by an aerosol method.
 17. A method as claimed in claim 10,wherein the colloidal sol or solution is applied to the membrane by aspin-coating method.
 18. A method of making a composite sheet, whichmethod comprises providing a porous anodic aluminum oxide membrane,pretreating a surface thereof with a solution of a silicate, borate orphosphate, applying to the pretreated surface a colloidal sol of aninorganic material or a solution of a polymeric precursor to aninorganic material so as to form thereon a gel layer, and drying andheating the gel to convert it to a microporous inorganic film ofsubstantially uniform pore size and substantially free of cracks andpinholes.
 19. A method as claimed in claim 18, wherein pretreatment iseffected by contacting the porous membrane with an aqueous solution ofthe silicate, borate or phosphate at a concentration of at least 1% byweight and a temperature of 0° to 50° C. for a contact time of from 1second to 10 minutes.
 20. A filter comprising a composite sheetcomprising a porous inorganic membrane and a microporous inorganic filmoverlying a surface of the membrane, wherein the porous inorganicmembrane is an anodic aluminum oxide membrane and the microporousinorganic film is of substantially uniform pore size and substantiallyfree of cracks and pin-holes.
 21. A method of making a composite sheet,which method comprises providing a porous anodic aluminum oxide membraneand a colloidal sol of an inorganic material or a solution of apolymeric precursor to an inorganic material, increasing the viscosityof the sol or solution by adding a relatively viscous miscible organicliquid.